For decades, scientists have devoted themselves to developing vanadium dioxide (VO2) films for thermochromic smart windows, but there remains a major challenge to their large-scale implementation owing to their limited solar modulation ability. Here, we show a new method for improving their solar modulation ability via increasing optical switching in the visible region upon phase transition. VO2-based composite films that exhibited reduced haze (haze(555) (nm) down from 46.5% to 2.2% @RT), improved luminous transmittance (up from 9.6% to 38.5% @RT) and enhanced solar modulation ability (Delta T-sol up from 8.1% to 16.9% @RT) have been fabricated by a convenient and controllable approach. With this method, the mean sizes of VO2 particles can be easily tuned from 150 nm to 40 nm by varying the preparation parameters. Via a combination of the optical transmittance spectrum and Mie calculations, we first observed the behavior of optical switching in the visible region upon phase transition, which is size-dependent. Moreover, distinctive size effects on variations in the band gap energy and specific interband transitions were studied based on the absorption spectra of VO2 composite films.